In Earth's oxygen rich environment, life forms have evolved that utilize the energy rich chemistries of oxygen to efficiently operate their life processes. But, in doing so they must constantly protect themselves from unwanted damaging free-radical oxidative reactions. These reactive oxygen species include superoxide anions, hydrogen peroxide and hydroxyl radicals, that can modify important cellular macromolecules and initiate or accelerate diseases. The formation of reactive oxygen species can occur as part of many cellular processes including mitochondrial respiration, immune cell responses, cell injury, heat, radiation, or from metabolism of drugs and other chemicals. Reactive oxygen species are thought to be involved in almost all disease processes and the ageing process. For example, peroxynitrite is found to be a particularly damaging agent in diabetic retinopathies.
Current evidence reveals a paradox in the manipulation of reactive oxygen species for therapies. For example, boosting the levels of reactive oxygen species has proved beneficial in some maladies while scavenging reactive oxygen species with antioxidants is effective in others.
Most of the attempted treatments of ocular afflictions and cancers with antioxidants have used oral administrations of antioxidants with moderate to poor results. The NIH AREDS study, completed in 2001, involving high oral doses of antioxidants and zinc, showed some inhibition of the progression of macular degeneration but no protective effect on cataract formation. High oral doses of vitamin C have been dismissed as being therapeutic in cancer treatment, because even massive oral doses of ascorbates could not yield the therapeutic level achieved by intravenous or intraperitoneal injection. Thus, there is a need for minimally invasive injectable or implantable sustained release antioxidant formulations to maintain intraocular or intratumoral therapeutic levels of antioxidants.